Cavity resonator



Nov. 16, 1948.

J. SCHELLENG 2,453,760

CAVITY RESONATOR Fil ed March 2, 1945 2 Sheets-Sheet 1 lNVE/VfO/Q By J C. SCHELLENG ATTORNEY Nov. 16, 1948- J. c. SCHELLENG CAVITY RESONATOR 2 Sheets-Sheet 2 Filed March 2, 1945 YA/VENTOR J. C. SCHELLENG TTORNEY Patented Nov. 16, 1948 John C. 'Schelleng,

Interlakcn, N. J assignor to Bell Telephone Laboratories,Incorporated, New

York, N. Y., a corporation of New York Application March 2, 1945, Serial No. 580,517 5 Claims. (Cl. 178-44) This invention relates to cavity resonators and more particularly to improvements in the energy transmission connections to cavity resonators.

An object of the invention is to provide a cavity Another object of the invention is to provide circuit connections for a cylindrical cavity resonator which will enhance oscillations .01 T1301 modes while discriminating against oscillations of TEn, TElm, TE2m and other transverse electric modes.

An additional object of :the invention is to provide circuit connections between a wave guide and a cavity resonator which shall discriminate against oscillations of undesired TM modes.

Cavity resonators serve in electrical microwave technique as selective devices in a manner somewhat analogous to that of tuned circuits at lower frequencies. However, because of their distributed reactances, large cavities are in general susceptible of a large number of diiferent modes The manner in which various modes of oscillation interact is attributable in part to coupling occurring at the entrance by which energy is supplied to the resonator. As an illustration, supspectively as TE and mode. Hence the factors causing damping in the undesired mode Will contribute to that in the desired mode.

agreement for oscillations of undesired modes.

In this specification, transverse electric and transverse magnetic modes will be designated re- TM, and in the case of right circular cylindrical resonators the sub- 180-degree phase changes in the radial and length directions. .Hence, TEoio mode oscillations in a right circular cylinder describe an oscillation having a standing wave pattern such that its electric vectors are transverse to the cylinder 7 axis; there is no change in phase circumferentially; there is a single half-Wave change in phase in radial direction and there are nine half- Wave changes in the longitudinal or axial direction.

In the drawing:

Fig. l shows in perspective a cavity resonator involving one embodiment of the invention;

Fig. 2 is a plan view of the lower end of the structure of Fig. 1;

Fig. 3 is a section along a vertical plane passing through the zigzag broken line 33 of Fig. 2.; and

Fig. 4 is a section along the line 4-4 of Fig. 2.

Referring to the drawing, a cylindrical cavity resonator l0 designed to operate in TEois mode is shownin Fig. 1.

seconds inch wide. The sion system which may serve as a feed line for the resonator is a wave guide 14 designed to transmit oscillation energy of the desired frequency. The radial coupling slots of apertures 82 have their length direction transverse to the electric vectors of the desired TEOl. mode in the resonator and in the wave guide coupling sections. They, accordingly, are relatively effective for transfer of oscillation energy of that mode but are relatively ineffective for transfer of energy of modes having radial electric vectors which are only slightly intercepted by the narrow slots. For oscillations of TEm mode, the four apertures excite the resonator in phase thus building up a strong symmetrical internal field. If a considerable number of apertures be used, effective discrimination may be had against modes having greater values of 'n than zero.

In passing from one of the exciting apertures to another in the direction in which the electric vector of the desired electromagnetic field extends no change of phase will be experienced. In this specification and the annexed claims the expressions in phase agreement or in phase shall connote that condition as distinguished from the situation in more complex systems where physically separated points are electrically at potentials differing by an integral number of wavelengths. Moreover, the expression gravest mode of oscillation as related to a particular dimension or surface of an electrical boundary shall be understood to connote the mode or the series of modes of oscillation which is most basic or is of the lowest frequency in so far as that particular dimension or surface is concerned. In the example of Fig. 1, in which a TEOIQ mode of oscillation is disclosed as a selected mode from the TEOln series, the gravest mode, referred to the end surface in which the coupling orifices occurs, is TEoi irrespective of the magnitude of n.

We might summarize as follows the behavior for the various modes of oscillation that require consideration in the case of a four aperture system used as a feed circuit for a cavity resonator. In general TEom oscillations are passed by radial slots. However, the mode corresponding to a particular value of m may be reduced by choice of the radius of the circle of slots. With respect to TMOm oscillations reduction may be had by use of narrow radial slots or by using as a feed circuit an external oscillation path providing a circumferential electric field or both of these expedients as in the system illustrated in the drawing. For T'EZk-Hgn or TM2r+1,m where k equals 0, l, 2, etc., so that the first subscript is odd, reduction may be had by using coupling slots arranged in symmetrical diametrical pairs. This is of particular importance in the case of the use of TEllm. oscillations where it is desired to suppress the equal frequency TMlm mode. For TEflm or TMzm oscillations the two pair or four aperture arrangement affords eifective reduction. For even values of the first subscript greater than 2 the four aperture feed does not effect a considerable reduction and it may be desirable to resort to a still greater number of apertures arranged in accordance with the principles of the invention.

The main wave guide M forks at I5 into two branches l6 and ll of equal height in the direction of the longitudinal axis of the resonator each branch being of a width approximately half the narrower or transverse dimension of the guide M. This device enables the main guide M to face a terminal impedance of the branches l6 and I1 which approximately matches its own. In turn,

each of the branches l6 and I1 is forked so as to terminate in a pair of terminal branches l8, H! or 20, 2| as the case may be.

It is desirable that excitations at the various apertures be in phase for oscillations of the desired mode. Consequently, for TEoi modes of oscillation with the apertures located as disclosed, the wave guide paths from the main transmission guide to the apertures l2 should be as nearly as possible electrically identical in wavelength.

"Notched configurations at the branching points 22, 23 and 24 assist in smoothing the transmission characteristics and in matching the parallel or joint impedance of the branches to that of the section from which they branch.

Tuning of the resonator Ill to a desired frequency may be effected by a well-known reciprocating piston structure indicated schematically at It will be apparent that the multiple coupling expedient of the invention enables a symmetrical excitation of a cavity resonator with consequent increase in correspondence of its actual performance with the theoretical or precalculated performance. It will also be obvious that the multiple coupling point expedient of the invention is an effective device for restricting the useful eifect of a cavity resonator to a desired mode of oscillation and for preventing dissipation of the energy of the desired mode by a reaction of unwanted oscillations of other modes.

What is claimed is:

1. An electrical resonator comprising a hollow substantially closed cylindrical chamber adapted for sustaining within its interior space electromagnetic oscillations of a natural resonance frequency of the chamber and of a TEOln mode, said chamber having plane circular end walls and having its interior space between said end walls sub-- stantially free of energy absorbing material or electrical conducting material, one of said end walls having a plurality of spaced apertures at points which are in phase agreement for oscillations of said resonance frequency and mode, and an oscillation path comprising a wave guide conduit respectively coupled to the field within said chamber at each of said apertures, said conduits each having equal rectangular cross-section with their greater transverse dimension arranged perpendicular to the plane of the apertured end wall, said conduits being all connected together at their ends remote from the respective aperture couplings to form a common rectangular transmission guide of the same major transverse dimension but of a greater dimension perpendicular to said transverse dimension.

2. An electrical resonator in accordance with claim 1 characterized in this that said apertures each consists of short narrow radial slits in the circular end wall of the resonator.

3. The structure of claim 2, wherein said apertures are located at equispaced points on a concentric circle where the TEOln mode field is strong relative to that at the center and, circumference of the end wall.

4. The structure of claim wherein said apertures are located at four equispaced points on a concentric circle of radius approximately=.4R, where R is the reference radius of the circular end wall.

5. An electrical resonator in accordance with claim 1 characterized in this that the conduits which merge to form a larger conduit are pro- ,vided at the junction point .with means for matching the impedances of the merging conduits to that of the larger conduit.

5 UNITED STATES PATENTS JOHN C. SCI'IE -LENG. Number ame Date 2,129,714 Southworth Sept. 13, 1938 REFERENCES CITED 6 2,206,923 SOIIthWOIth July 9, 1940 The following references are of record in the McArthur May 1942 file of this patent:

Bowen Out. 1, 1946 

